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The CO2 complexes with HOO and HO in argon matrices
Chemical Physics 265 (2001) 323±333
www.elsevier.nl/locate/chemphys
The CO2 complexes with HOO and HO in argon matrices Thomas Svensson a, Bengt Nelander a,*, Gunnar Karlstr om b a
Thermochemistry, Chemical Physics, Chemical Center, P.O. Box 124, S-22100 Lund, Sweden b Theoretical Chemistry, Chemical Center, P.O. Box 124, S-22100 Lund, Sweden Received 30 November 2000; in ®nal form 21 February 2001
Abstract Complexes of the peroxy radical and the hydroxy radical with CO2 have been investigated with FTIR spectroscopy. The peroxy radical±CO2 complex was formed on the surface of growing argon matrices by codepositing peroxy radicals and CO2 . The hydroxyl radical±CO2 complex was formed on photolysis (266 nm) of the matrix isolated peroxy radical± CO complex. The bands were identi®ed from concentration dependencies, H to D shifts and by photolysis experiments. In the photolysis experiments the correlation coecients between data series ± decay and growth curves ± and the degree of photolysis of the bands were calculated. Based on these data the bands were then assigned. The correlation coecients also provided information about reactant/product relations. Ab initio calculations on the SCF level have been carried out in order to investigate the relations between complex structure and spectral shifts. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The matrix isolation method has been used to study infrared spectra of numerous complexes between stable molecules. Complexes between free radicals and stable molecules have been observed in matrix isolation studies of free radicals, when the method of preparation of the radical has favored their formation. We have started a study of the complexes between the peroxy radical and small, stable molecules [1±3]. This paper presents a study of the peroxy radical complex with carbon dioxide. Hydroxyl radical complexes in the gas phase have been studied by Lester and coworkers [4].
*
Corresponding author. Fax: +46-46-222-4119. E-mail address: [email protected] (B. Nelander).
The hydroxyl radical in noble gas matrices was ®rst studied in ¯uorescence [5]; its infrared absorption has turned out to be rather dicult to observe. For a long time there have been two con¯icting assignments for the fundamental vibration of OH in argon matrices in the literature. Acquista et al. [6] photolyzed water (H2 16 O, H2 18 O, D2 16 O and D2 18 O) on the surface of growing argon matrices and assigned a pair of bands at 3453.2 and 3428.2 cm 1 to free OH in solid argon. Cheng et al. [7] codeposited hydrogen atoms (H,D) and oxygen atoms (16 O, 18 O) in argon matrices. They also produced OH by reaction of hydrogen atoms with O3 or NO2 in the gas phase and deposited the reaction products in argon matrices. They assigned a band at 3548.2 cm 1 to free OH in an argon matrix. The combined ultraviolet emission and infrared absorption studies of the products of hydrogen peroxide photodecomposition by the Helsinki group have shown that the
0301-0104/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 0 1 ) 0 0 3 2 1 - 4
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higher assignment of the OH fundamental is correct [8]. The method of preparation of the hydroxyl radical, which produced the lower assignment, suggests the possibility that this band is in fact due to a hydrogen bonded complex between a hydroxyl radical and a water molecule. A few studies of hydroxyl radical complexes in matrices have been published and in all cases the complexes have been prepared by photoisomerization. Fushitani et al. [9] photolyzed HI in para hydrogen matrices which contained CO2 . A OC±HO complex formed, but the hydroxyl radical reacted with the matrix to form water. McInnis and Andrews formed a HO±HF complex by irradiation of the H2 O±FF complex [10]. In this paper we report a study of the OCO±HO complex which we obtained by irradiation of the OC±HOO complex. As will be seen further on in the text, some of the experimentally obtained results are rather hard to understand from simple reasoning, and in order to shed some light on this we have chosen to perform a set of quantum chemical ab initio calculations on the SCF level. The purpose of these calculations is not to try to reproduce the experimental results, but rather to investigate trends in the shifts of the intramolecular vibrations due to the formation of the complexes. 2. Experimental section Hydrogen (AGA), deuterium (LÕAir liquide, 99.7% D), O2 (LÕAir liquide, 99.9995%), CO (Aga 4.7), CO2 (LÕAir Liquide 99.995% pure), Ar (LÕAir Liquide 99.9995%) were used as delivered. Deionized water was degassed before use. Gas mixtures were prepared by standard manometric techniques. Ar/H2 or Ar/D2 gas mixtures were passed through a microwave discharge. The Ar/H2 and Ar/D2 ratios were varied between 38 and 75. A second mixture of Ar and O2 and CO or CO2 was passed through a separate line. The Ar/O2 ratio was varied between 38 and 75 and the Ar/CO or Ar/CO2 ratios between 150 and 750. Directly after deposition, we did not observe any eect of H- or D-atoms on CO or CO2 . The CO2 ±water complex was prepared by codepositing water and CO2 . Then one volume with Ar and water and second
with Ar and CO2 were prepared. The Ar/water ratio was kept at 75 and the Ar/CO2 ratio at 750. Nupro needle valves were used to regulate the ¯ow (10 mmol/h, 2 h deposition time) from the two volumes. The matrices were deposited on a CsI window at 17 K, cooled by a Leybold RDK 10-320 closed cycle cryocooler. They were irradiated, in steps, with 266 nm radiation from a quadrupled Continuum NY 20 C YAG laser. The irradiation times were chosen long enough to give clearly measurable absorption changes, but so short that only fractions of the decomposing bands were eliminated. Infrared spectra were recorded between 500 and 4000 cm 1 at 0.5 cm 1 resolution with a Bruker 113v FTIR spectrometer. Some of the CO2 bands were found to have widths that were much less than 0.5 cm 1 [11]. Therefore spectra were also recorded at 0.1 cm 1 resolution in some of the experiments. All spectra were ratioed against a background spectrum recorded before deposition. 3. Computational details and results All calculations reported in this work are performed with the M O L C A S program system [12]. The basis set is a 14s, 9p and 4d primitive basis set for O and C, which has been contracted using a general contraction scheme to 5s, 4p and 2d basis functions [13]. For hydrogen a 8s, 3p primitive basis set was contracted to 4s and 2p basis functions [13]. The closed shell molecules were studied using the closed shell Hartree Fock SCF program in the M O L C A S program system and the open shell system were studied using the corresponding RASSCF program as an open shell SCF program. For each of the systems studied a full geometry optimization starting from a structure which we believed to be consistent with the experimental observations. The resulting structures are shown in Figs. 6 and 7. At the optimal geometry a vibrational spectrum was calculated using the harmonic approximation. The Hessian was calculated using an analytical procedure. In Table 1 we present the calculated total energies (in a.u.) for the dierent systems studied, together with the formation en-
T. Svensson et al. / Chemical Physics 265 (2001) 323±333 Table 1 Calculated energies (see text) Total energy (H) OH HOO CO2 OCO±HO OCO±HOO
75.421320 150.239574 187.717025 263.140962 337.959965
Energy of formation (kJ/mol)
6.87 8.84
ergy for the complexes in kJ/mol. In Table 6 we show the calculated complex shifts of the fundamental vibrational frequencies for the studied systems. The results will be discussed in connection with the presentation of the experimental results.
4. Assignment 4.1. Nomenclature In complexes of the type studied here, the intramolecular vibrations of the complex components retain their original character in the complex. Therefore, the perturbed ith fundamental of A in a complex with B will be denoted as mi (A±B). When the isotopic composition of B is immaterial, the peroxy radical will be denoted as perox and water as aq.
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4.1.1. H2 O±CO2 complex The method used to prepare peroxy radicals always produces signi®cant amounts of water [1]. It was therefore necessary to study the water complex of carbon dioxide. The IR spectrum of the matrix isolated water complex of carbon dioxide has as far as we know only been studied in solid nitrogen by Fredin et al. [16] and in solid oxygen by Tso and Lee [17]. There is good qualitative accord between our results and those of Refs. [16,17]. The results are collected in Table 2. When water and carbon dioxide were simultaneously present in the matrix, two bands at 656.0 and 668.0 cm 1 increased signi®cantly relative to the three bands of free CO2 at 662.0, 663.5 and 663.9 cm 1 . These bands were assigned to the complex between H2 O±CO2 . The same band positions were observed with D2 O instead of H2 O. A band near 2340.5 cm 1 was assigned to m3 (O2 C± OH2 ). m1 (H2 O±CO2 ) was observed at 3632.7 cm 1 , m2 (H2 O±CO2 ) at 1593.1 cm 1 and m3 (H2 O±CO2 ) at 3732.9 cm 1 . 4.1.2. HOO±CO2 complexes (1:1) When peroxy radicals and CO2 were codeposited on a matrix we observed a set of new bands close to the peroxy radical and carbon dioxide fundamentals. Based on concentration dependencies we assign the following bands to one or more
Table 2 Observed bands of the complex of water with CO2 in solid argon (cm 1 ) H2 O(D2 O), m1 H2 O(D2 O), m2 H2 O(D2 O), m3 CO2 , m1 CO2 , m2
H2 O
D2 O
CO2
H2 O±CO2
D2 O±CO2
3638.0 1589.1 3734.3
2657.7 1174.6 2771.1
± ± ±
3632.7 1593.1 3732.9
1177.4 2770.9
656.0 668.0
656.0 668.0
662.2a 663.5 663.9
a
CO2 , m3
2345.1a 2339.3 2340.5
2340.5
2340.5
13
2279.5a 2274.0 2275.0
2275.0
2275.0
CO2 , m3
Stable site.
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OOH±OCO 1:1 complexes: 3372.1, 3376.4, 1410.1, 1415.6, 1109.5, 1110.9, 656.2, 668.1, 2339.9, 2340.1, 2274.4, 2274.6. In experiments with DOO we assign bands at 2499.5, 2501.7, 1037.9, 1041.8, 655.9, 668.1, 2338.8, 2339.0, 2273.5, 2273.7 to a OOD±OCO complex (1:1). The rate of photolysis varied between the bands. In fact from their photolysis rates they could be assigned to two dierent complexes, I and II. The bands assigned to I had a photolysis rate that was twice as high as bands of
II. We assign the bands at 3376.4, 1415.6, 1110.9, 2339.9 and 2340.1 cm 1 (13 C: 2274.4 and 2274.6 cm 1 ) to I (D: 2501.7, 1041.8, 2338.8, 2339.0) and the bands at 3372.1, 1410.1, 1109.5, 656.2, 668.1 cm 1 to II (D: 2499.5, 1037.9, 655.9, 668.1). The assignments are collected in Table 3. m1 (OOH± OCO) (I, II) at 3376.4 and 3372.1 cm 1 and m1 (OOD±OCO) (I, II) at 2499.5 and 2501.7 cm 1 are shown in Fig. 1. Fig. 2 shows m3 (O 13 CO±HOO) and m3 (O 13 CO±HOO).
Table 3 Observed bands of the complex of the peroxy radical and hydroxyl radical with CO2 (cm 1 ) HOO HOO, m1 HOO, m2 HOO, m3 HO, m CO2 , m2
3413.0 1388.9 1100.9
HO
3548.2a
CO2
662.2b
OH±OCO
3557.0 658.9
HOO±CO2 (I)
HOO±CO2 (II)
3376.4 1415.6 1110.9
3372.1 1410.1 1109.5 656.2 668.1
663.5 663.9 2345.1b
CO2 , m3
2350.1
2339.9 2340.1
2284.5
2274.4 2274.6
OD±OCO
DOO±CO2 (I)
DOO±CO2 (II)
2501.7 1041.8
2499.5 1037.9
2339.3 2340.5 13
2279.5b
CO2 , m3
2274.0 2275.0 DOO DOO, m1 DOO, m2 DOO, m3 DO, m CO2 , m2
2530.2 1020.3 1122.9
DO
2616.1a
CO2
662.2b
2621.4 658.6
655.9 668.1
663.5 663.9 CO2 , m3
2345.1b
2350.7
2338.8 2339.0
2285.0
2273.5 2273.7
2339.3 2340.5 13
CO2 , m3
2279.5b 2274.0 2275.0
a b
Ref. [7]. Stable site.
T. Svensson et al. / Chemical Physics 265 (2001) 323±333
Fig. 1. The HO and DO stretching vibration of the peroxy radical±CO2 complexes; resolution, 0.1 cm 1 ; x-axis, cm 1 ; upper curves, spectra before photolysis, ratioed against background spectra; lower curves, spectra after 60 min of photolysis, ratioed against spectra before photolysis; the curves have been shifted vertically for clarity. (a) m1 (HOO±CO2 ) (II) at 3372.1 cm 1 and m1 (HOO±CO2 ) (I) at 3376.4 cm 1 , (b) m1 (DOO±CO2 ) (II) at 2499.5 cm 1 and m1 (DOO±CO2 ) (I) at 2501.7 cm 1 .
The CO2 bands at 663.5, 663.9, 2339.3, 2340.5, 2274.0 and 2275.0 cm 1 have been assigned to CO2 in a metastable trapping site by Schriver et al. [11]. These bands decrease in intensity during photolysis at the same rate as the bands of complex II. Fig. 2 illustrates this for m3 (O 13 CO), note that the stable site is unaected. When a matrix containing only CO2 was irradiated the bands of the metastable site were unaected. Possibly the photodecomposition of peroxy radicals and peroxy radical complexes deposit enough energy in the matrix to anneal away the metastable site. The
327
Fig. 2. The 13 CO2 , m3 , stretching vibration of the peroxy radical±CO2 complexes; resolution, 0.1 cm 1 ; x-axis, cm 1 ; upper curves, spectra before photolysis, ratioed against background spectra; lower curves, spectra after 60 min of photolysis, ratioed against spectra before photolysis; the curves have been shifted vertically for clarity. The CO2 bands at 2274.0 and 2275.0 cm 1 have been assigned to CO2 in a metastable trapping site by Schriver et al. [11]: (a) m3 (O2 C±OOH) (I) at 2274.4 and 2274.6 cm 1 , (b) m3 (O2 C±OOD) (I) at 2273.5 and 2273.7 cm 1 .
metastable site is also eliminated when the matrix is warmed above 30 K [11]. 4.1.3. The complex between HO and CO2 When matrices containing the OC±HOO complex were irradiated, bands appeared at 658.9, 2350.1, (13 C: 2284.5) and 3557.0 cm 1 . The matrices with matrix isolated peroxy radicals and CO were irradiated in steps, with a spectrum recorded after each step. In order to identify relations
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between reactant±product bands in the spectra, correlation coecients were calculated for each pair of bands using standard statistical methods. The M A T L A B â commands corrcoef x; y or corrcoef X were used. If the growth of a product band is due to the decomposition of a precursor, the correlation coecients of their absorption bands are ideally 1. The bands in the OOH±CO complex proved to be anti-correlated with the bands at 658.9, 2350.1 and 3557.0 cm 1 . The correlation coecients were calculated for six matrices with close agreement between the matrices. With H and 0.1 cm 1 resolution the correlation coecients varied between 0.983 and 0.998. In OC±DOO experiments, the corresponding bands appeared at 659.7, 2350.7, (13 C: 2285.0) and 2621.4 cm 1 (Figs. 3±5). The bands at 658.9 and at 2350.1 cm 1 are close to the infrared active fundamentals of carbon dioxide, showing that the photoproduct contains a perturbed CO2 molecule. The 2284.5 cm 1 band is clearly the 13 CO2 band corresponding to 2350.1 cm 1 . Note the small but signi®cant H- to D-shifts of the carbon dioxide fundamentals which show that the carbon dioxide molecule is perturbed by a species which contains hydrogen. Since the growth rate suggests that the photoproduct forms from OC±HOO it seems clear that the perturbing species is a hydroxyl radical. The position of the third band, 3557.0 cm 1 (D: 2621.4 cm 1 ) is compatible with this assignment. 4.1.4. Unassigned bands When matrices with matrix isolated peroxy radicals and CO were irradiated we observed weak bands at 1862.5 and 1863.8 cm 1 (D: 1925.0, 849.4 cm 1 ). These bands are probably due to HCO [14]. A band near 1903.7 cm 1 (1905.2 cm 1 in Dexperiments) was formed with a high rate in CO2 HOO experiments. It is possibly due to the complex HO±CO3 . Weissberger et al. [15] observed a set of new bands when solid CO2 matrix with dispersed O3 was irradiated with UV light. The new bands were assigned to CO3 . Traces of O3 are always present in our matrices, possibly since O atoms form from H2 O present in the microwave discharge. Bands which we assign to CO3 were observed at 2043.4 (strongest) and 2037.3 cm 1 .
Fig. 3. The HO and DO stretching vibration of the hydroxy radical±CO2 complex; resolution, 0.1 cm 1 ; x-axis, cm 1 ; upper curves, spectra before photolysis, ratioed against background spectra; lower curves, spectra after 60 min of photolysis, ratioed against spectra before photolysis; the curves have been shifted vertically for clarity. (a) m(OH±OCO) at 3557.0 cm 1 , (b) m(OD± OCO) at 2621.4 cm 1 .
During the ®rst minutes of photolysis, the bands grow at a high rate. It is followed by a growth at slower rate. Small amounts of CO3 were also observed by irradiating a matrix with codeposited peroxy radicals and CO. In addition to the bands assigned to the peroxy radical and the hydroxy radical complexes, we observed a number of relatively weak bands with H- to D-shifts near the fundamentals of HOO, HO and CO2 which are not assigned. Most of them were also observed in experiments with codeposition of peroxy radicals and CO2 . They are collected in Table 4.
T. Svensson et al. / Chemical Physics 265 (2001) 323±333
Fig. 4. The CO2 , m2 , stretching vibration of the hydroxy radical±CO complex; resolution, 0.1 cm 1 ; x-axis, cm 1 ; upper curves, spectra before photolysis, ratioed against background spectra; lower curves, spectra after 60 min of photolysis, ratioed against spectra before photolysis; the curves have been shifted vertically for clarity. (a) m2 (OCO±HO) at 658.9 cm 1 , (b) m2 (OCO±DO) at 658.6 cm 1 .
By irradiating (at 266 nm) the matrix with codeposited HOO and CO2 , we observed a set of bands that grew near the HOO vibrational fundamentals of the HOO±CO2 complexes. After approximately 30 min of irradiation they started to photodecompose. The bands were observed at 3370.9 cm 1 (2498.5 cm 1 with DOO), at 1411.1 cm 1 (1039.1 cm 1 with DOO) and at 1108.4 cm 1 (no corresponding band was observed with DOO).
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Fig. 5. The 13 CO2 , m3 , stretching vibration of the hydroxy radical±CO2 complexes; resolution, 0.1 cm 1 ; x-axis, cm 1 ; upper curves, spectra before photolysis, ratioed against background spectra; lower curves, spectra after 60 min of photolysis, ratioed against spectra before photolysis; the curves have been shifted vertically for clarity. The band at 2279.5 cm 1 is due to 13 CO2 (stable site); the band at approximately 2281.5 cm 1 , see Table 4: (a) m3 (O 13 CO±HO) at 2284.5 cm 1 , (b) m3 (O 13 CO±DO) at 2285.0 cm 1 .
We observed a set of bands that grew in the HO and CO2 (m2 and m3 ) regions in experiments with matrix isolated peroxy radicals and CO or CO2 . They were observed at 3428.9, 655.1, 666.1, 2346.6 (13 C: 2281.3) cm 1 (DOO: 2543.3, 654.8, 666.2, 2346.8 (13 C: 2281.4 cm 1 )). The growth rate was approximately the same for the bands and anticorrelated with the decrease of the peroxy radical bands. It is clear that the bands at 655.1 and 666.1 belong to the same specie but we could not conclude de®nitively that the bands at 3428.9, 2346.6
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Table 4 Observed bands with H- to D-shifts H (cm 1 )
D (cm 1 )
Experiment
Notes
2336.5 2335.4a
2335.9 2335.0a
Peroxy radicals CO2 ''
Photolysis rate: >photolysis rate (HOO±CO2 ) ''
2334.4 2337.1 3370.9
2334.5 2336.4 2498.5
Peroxy radicals CO2 Peroxy radicals CO Peroxy radicals CO2
1411.1 1108.4
1039.1
'' ''
Photolysis rate:
3428.9
2543.3
655.1 666.1 2346.7 2281.3
654.8 666.2 2346.8 2281.4
Peroxy radicals CO2 and hydroxyl radicals CO2 '' '' '' ''
2342.1
2342.0
2276.7
2276.6
b
3382
b
2507
Grow when the matrix is irradiated '' '' '' ''
Peroxy radicals CO2 and hydroxyl radicals CO2 ''
Grow when the matrix is irradiated ''
Peroxy radicals CO2
Photolysis rate: photolysis rate (HOO±CO2 )
a
Satellite. b Broad band.
and 2281.3 cm 1 also belong to this specie. One band at 2342.1 (13 C: 2276.7) cm 1 (DOO: 2342.0 (13 C: 2276.6) cm 1 ) was formed in the same experiments as the set of bands above, but at a somewhat higher rate. A band at 3382 cm 1 (2507.1 cm 1 in Dexperiments) was only observed when peroxy radicals and CO2 were present at the same time in the matrix and obviously it is a product formed by codepositing peroxy radicals and carbon dioxide (Fig. 2). Its concentration dependence diered from the 1:1 complex bands. The photolysis rate was approximately half of the rate of HOO±CO2 (II). A band at 2336.5 cm 1 was observed, with a small satellite at 2335.4 cm 1 . This band was almost eliminated from the matrix by 5 min of irradiation (266 nm). In D-experiments the band was observed at 2335.9, with a small satellite at 2335.0 cm 1 . The position of the band and the sensitivity to photolysis indicate the presence of CO2 and O3 . The band could be due to a three component species like water±CO2 ±O3 .
5. Discussion 5.1. H2 O±CO2 complex The results from the study of the water±CO2 complex is in agreement with an earlier studies [16,17]. We observe the splitting of the bending mode in CO2 (12.0 cm 1 ). This is consistent with a structure where the water oxygen binds to the carbon dioxide carbon, as found by Peterson and Klemperer [18] and earlier predicted by J onsson et al. [19]. Note also that the antisymmetric stretch of carbon dioxide is shifted towards lower frequencies. In the OCO±HCl complex, where HCl forms a hydrogen bond to a carbon dioxide oxygen, the antisymmetric stretch is blue shifted. The shifts of the water fundamentals are typical for a complex with water as a lone pair donor. 5.2. Peroxy radical±CO2 complexes HOO±CO2 (I, II): The complex shifts of HOO in the two OCO±HOO complexes dier by rather
T. Svensson et al. / Chemical Physics 265 (2001) 323±333
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Table 5 Bands of some selected CO2 complexes (cm 1 ) Free
ClH±OCOa
HCl, m HO, me HOO, m1 HOO, m2 HOO, m3 H2 O, m1 H2 O, m2 H2 O, m3 CO2 , m1 CO2 , m2
2870.8 3548.2 3413.0 1388.9 1100.9 3638.0 1589.1 3734.3
2854.1
662.2
658
656.0 668.0
658.9
CO2 , m3 13 CO2 , m3
2345.1 2279.5
2346.1 2280.9
2340.5 2275.0
2350.1 2284.5
d
H2 O±CO2 b
OH±OCOc 3557.0
3632.7 1593.1 3732.9
HOO±CO2 (I)c
HOO±CO2 (II)c
3376.4 1415.6 1110.9
3372.1 1410.1 1109.5
656.2 668.1 2274.4 2274.6
a
Ref. [18]. Taken from Table 2. c Taken from Table 3. d Ref. [18]. e Ref. [7]. b
insigni®cant amounts (Table 3). The only dierence between the two complexes seems to be their photodecomposition rates. We therefore believe that they are identical complexes, trapped in different sites. The bending vibration of CO2 bound to a peroxy radical splits into two components in the same way as in the carbon dioxide water complex (Table 5). The water carbon dioxide complex is a planar C2v complex, with the water oxygen close to the carbon dioxide carbon [18]. We therefore expect that the terminal oxygen of HOO is close to the carbon dioxide carbon. We note that for the linear OCO±HCl complex, the carbon dioxide bend is a single band at 658 cm 1 [20]. The OH shifts of the two complexes, 36.6 and 40.9 cm 1 , are approximately twice the HCl shift of OCO±HCl. The observation that the OH shift is larger than the HCl shift of the CO2 ±HCl complex supports the assumption of an additional interaction in the HOO complex [3]. The complexes are thus expected to have a cyclic structure, probably planar, with the peroxy radical hydrogen bonded to a lone pair of an oxygen atom and turned around its HO bond so that its terminal oxygen is as close as possible to the carbon dioxide carbon.
As can be seen from Fig. 6, the calculated structure of the peroxy radical carbon dioxide complex has a hydrogen bond from the peroxy radical to a carbon dioxide oxygen. The terminal
Fig. 6. The calculated structure of the peroxy radical carbon dioxide complex (see text).
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T. Svensson et al. / Chemical Physics 265 (2001) 323±333
Table 6 Calculated and experimental complex shifts (cm 1 ) (OCO± HOO type II was used with one exception) Calculated m2 (OCO±HOO) m3 (OCO±HOO) m1 (HOO±OCO) m2 (HOO±OCO) m3 (OOH±OCO) m(OH±OCO) m2 (OCO±HO) m3 (OCO±HO) a
2.7 2.9
Experimental 5.9 6.0
2.0 10.7 25.9 3.9
5.0 40.9 21.2 8.6
0.0 3.5 4.3
8.8 3.3 5.0
a
From type OCO±HOO type I.
oxygen of the peroxy radical is turned in the direction of the carbon dioxide. The calculated structure is thus in general agreement with the structure suggested by the observed complex shifts. The calculated complex shifts (Table 6) are in approximate agreement with the observed shifts but the calculated split of the carbon dioxide is almost zero, suggesting a too weak interaction between the terminal oxygen of the peroxy radical and the carbon dioxide carbon. A similar problem was encountered in a calculation on the interaction between a peroxy radical and chlorine dioxide [21]. Here the calculation produced a too strong hydrogen bond from the peroxy radical to an oxygen atom but a too weak interaction between the terminal oxygen of the peroxy radical and the chlorine atom. From theoretical considerations one would in general expect a too weak carbon±oxygen interaction in the calculations, since the attractive dispersion interaction is lacking in the SCF approximation. 5.3. The hydroxyl radical±CO2 complex OH±OCO: The hydroxyl radical carbon dioxide complex has a single strong OCO bend at 658.9 cm 1 , which shifts to 658.6 cm 1 in D-experiments. The antisymmetric stretch of OCO is blue shifted 5.0 cm 1 with respect to the major site fundamental of carbon dioxide. The spectral changes of complexed carbon dioxide are thus very similar to the shifts of carbon dioxide complexed with HCl
Fig. 7. The calculated structure of the hydroxyl radical carbon dioxide complex (see text).
(Dm2 4 cm 1 and Dm3 1:0 cm 1 ). We therefore believe that the complex is close to linear with the hydroxyl radical forming a weak hydrogen bond to a carbon dioxide oxygen. For this complex structure one expects a signi®cant red shift of the hydroxyl radical fundamental. Instead we observe a small blue shift. The exact position of the hydroxyl radical fundamental in solid argon is uncertain, since there is a possibility that the observed band is due to hydroxyl radicals interacting weakly with some impurity. The size of the shift is therefore uncertain but it is at most a few wavenumbers. As can be seen from Fig. 7, the ab initio calculation predicts an almost linear complex, in agreement with the discussion above. The calculated complex shifts are quite close to the observed. Even the most surprising experimental result, the practically nonexistent shift of the hydroxyl radical, is reproduced. We ®nally note that in the HO±HF complex [10], where OH accepts a hydrogen bond from HF, the OH stretching frequency is red shifted by 10.3 cm 1 . Acknowledgements This work was supported by the Swedish Natural Science Research Council. The Carl Trygger Foundation ®nanced an upgrading of the FTIR Spectrometer. The YAG Laser was ®nanced by a grant from FRN. References [1] [2] [3] [4]
B. Nelander, J. Phys. Chem. A 101 (1997) 9092. A. Engdahl, B. Nelander, Chem. Phys. 249 (1999) 215. T. Svensson, B. Nelander, Chem. Phys. 262 (2000) 445. M.W. Todd, D.T. Todd, M.I. Lester, J. Phys. Chem. A 104 (2000) 6532. [5] J. Goodman, L.E. Brus, J. Chem. Phys. 67 (1977) 4858.
T. Svensson et al. / Chemical Physics 265 (2001) 323±333 [6] N. Acquista, L.J. Schoen, D.R. Lide Jr., J. Chem. Phys. 48 (1968) 1534. [7] B.M. Cheng, Y.P. Lee, J.F. Ogilvie, Chem. Phys. Lett. 151 (1988) 109. [8] L. Khriachtchev, M. Pettersson, S. Touminen, M. R as anen, J. Chem. Phys. 107 (1997) 7252. [9] M. Fushitani, T. Shida, T. Momose, M. Rasanen, J. Phys. Chem. 104 (2000) 3635. [10] T.C. McInnis, L. Andrews, J. Phys. Chem. 96 (1992) 2051. [11] A. Schriver, L. Schriver-Mazzuoli, A.A. Vigasin, Vibr. Spect. 23 (2000) 83. [12] K. Andersson, M.R.A. Blomberg, M.P. Fulscher, G. Malmqvist, P. Neogrady, Karlstr om, R. Lindh, P.-A. B.O. Roos, A.J. Sadlej, L. Seijo, L. Serrano-Andres, P.E.M. Siegbahn, P.O. Widmark, M O L C A S , version 4.0., Department of Theoretical Chemistry, Lund University, Sweden, 1997.
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[13] K. Pierloot, B. Dumez, P.-O. Widmark, B.O. Roos, Theor. Chim. Acta 77 (1990) 291. [14] M.E. Jacox, Vibrational and electronic energy levels of polyatomic transient molecules, J. Phys. Chem. Ref. Dat Monogr., New York 3 (1994). [15] E. Weissberger, W.H. Breckenbridge, H. Taube, J. Chem. Phys. 47 (1967) 1764. [16] F. Fredin, B. Nelander, G. Ribbeg ard, Chemica Scripta 7 (1974) 11. [17] T.-L. Tso, E.K.C. Lee, J. Phys. Chem. 89 (1985) 1612. [18] K.I. Peterson, W. Klemperer, J. Chem. Phys. 80 (1984) 2439. [19] B. J onsson, G. Karlstr om, H. Wennerstr om, Chem. Phys. Lett. 30 (1975) 58. [20] N. Fourati, B. Silvi, J.P. Perchard, J. Chem. Phys. 81 (1984) 4737. [21] T. Svensson, B. Nelander, A. Bernhardsson, G. Karlstr om, J. Phys. Chem. A 103 (1999) 4432.
www.elsevier.nl/locate/chemphys
The CO2 complexes with HOO and HO in argon matrices Thomas Svensson a, Bengt Nelander a,*, Gunnar Karlstr om b a
Thermochemistry, Chemical Physics, Chemical Center, P.O. Box 124, S-22100 Lund, Sweden b Theoretical Chemistry, Chemical Center, P.O. Box 124, S-22100 Lund, Sweden Received 30 November 2000; in ®nal form 21 February 2001
Abstract Complexes of the peroxy radical and the hydroxy radical with CO2 have been investigated with FTIR spectroscopy. The peroxy radical±CO2 complex was formed on the surface of growing argon matrices by codepositing peroxy radicals and CO2 . The hydroxyl radical±CO2 complex was formed on photolysis (266 nm) of the matrix isolated peroxy radical± CO complex. The bands were identi®ed from concentration dependencies, H to D shifts and by photolysis experiments. In the photolysis experiments the correlation coecients between data series ± decay and growth curves ± and the degree of photolysis of the bands were calculated. Based on these data the bands were then assigned. The correlation coecients also provided information about reactant/product relations. Ab initio calculations on the SCF level have been carried out in order to investigate the relations between complex structure and spectral shifts. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The matrix isolation method has been used to study infrared spectra of numerous complexes between stable molecules. Complexes between free radicals and stable molecules have been observed in matrix isolation studies of free radicals, when the method of preparation of the radical has favored their formation. We have started a study of the complexes between the peroxy radical and small, stable molecules [1±3]. This paper presents a study of the peroxy radical complex with carbon dioxide. Hydroxyl radical complexes in the gas phase have been studied by Lester and coworkers [4].
*
Corresponding author. Fax: +46-46-222-4119. E-mail address: [email protected] (B. Nelander).
The hydroxyl radical in noble gas matrices was ®rst studied in ¯uorescence [5]; its infrared absorption has turned out to be rather dicult to observe. For a long time there have been two con¯icting assignments for the fundamental vibration of OH in argon matrices in the literature. Acquista et al. [6] photolyzed water (H2 16 O, H2 18 O, D2 16 O and D2 18 O) on the surface of growing argon matrices and assigned a pair of bands at 3453.2 and 3428.2 cm 1 to free OH in solid argon. Cheng et al. [7] codeposited hydrogen atoms (H,D) and oxygen atoms (16 O, 18 O) in argon matrices. They also produced OH by reaction of hydrogen atoms with O3 or NO2 in the gas phase and deposited the reaction products in argon matrices. They assigned a band at 3548.2 cm 1 to free OH in an argon matrix. The combined ultraviolet emission and infrared absorption studies of the products of hydrogen peroxide photodecomposition by the Helsinki group have shown that the
0301-0104/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 0 1 ) 0 0 3 2 1 - 4
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T. Svensson et al. / Chemical Physics 265 (2001) 323±333
higher assignment of the OH fundamental is correct [8]. The method of preparation of the hydroxyl radical, which produced the lower assignment, suggests the possibility that this band is in fact due to a hydrogen bonded complex between a hydroxyl radical and a water molecule. A few studies of hydroxyl radical complexes in matrices have been published and in all cases the complexes have been prepared by photoisomerization. Fushitani et al. [9] photolyzed HI in para hydrogen matrices which contained CO2 . A OC±HO complex formed, but the hydroxyl radical reacted with the matrix to form water. McInnis and Andrews formed a HO±HF complex by irradiation of the H2 O±FF complex [10]. In this paper we report a study of the OCO±HO complex which we obtained by irradiation of the OC±HOO complex. As will be seen further on in the text, some of the experimentally obtained results are rather hard to understand from simple reasoning, and in order to shed some light on this we have chosen to perform a set of quantum chemical ab initio calculations on the SCF level. The purpose of these calculations is not to try to reproduce the experimental results, but rather to investigate trends in the shifts of the intramolecular vibrations due to the formation of the complexes. 2. Experimental section Hydrogen (AGA), deuterium (LÕAir liquide, 99.7% D), O2 (LÕAir liquide, 99.9995%), CO (Aga 4.7), CO2 (LÕAir Liquide 99.995% pure), Ar (LÕAir Liquide 99.9995%) were used as delivered. Deionized water was degassed before use. Gas mixtures were prepared by standard manometric techniques. Ar/H2 or Ar/D2 gas mixtures were passed through a microwave discharge. The Ar/H2 and Ar/D2 ratios were varied between 38 and 75. A second mixture of Ar and O2 and CO or CO2 was passed through a separate line. The Ar/O2 ratio was varied between 38 and 75 and the Ar/CO or Ar/CO2 ratios between 150 and 750. Directly after deposition, we did not observe any eect of H- or D-atoms on CO or CO2 . The CO2 ±water complex was prepared by codepositing water and CO2 . Then one volume with Ar and water and second
with Ar and CO2 were prepared. The Ar/water ratio was kept at 75 and the Ar/CO2 ratio at 750. Nupro needle valves were used to regulate the ¯ow (10 mmol/h, 2 h deposition time) from the two volumes. The matrices were deposited on a CsI window at 17 K, cooled by a Leybold RDK 10-320 closed cycle cryocooler. They were irradiated, in steps, with 266 nm radiation from a quadrupled Continuum NY 20 C YAG laser. The irradiation times were chosen long enough to give clearly measurable absorption changes, but so short that only fractions of the decomposing bands were eliminated. Infrared spectra were recorded between 500 and 4000 cm 1 at 0.5 cm 1 resolution with a Bruker 113v FTIR spectrometer. Some of the CO2 bands were found to have widths that were much less than 0.5 cm 1 [11]. Therefore spectra were also recorded at 0.1 cm 1 resolution in some of the experiments. All spectra were ratioed against a background spectrum recorded before deposition. 3. Computational details and results All calculations reported in this work are performed with the M O L C A S program system [12]. The basis set is a 14s, 9p and 4d primitive basis set for O and C, which has been contracted using a general contraction scheme to 5s, 4p and 2d basis functions [13]. For hydrogen a 8s, 3p primitive basis set was contracted to 4s and 2p basis functions [13]. The closed shell molecules were studied using the closed shell Hartree Fock SCF program in the M O L C A S program system and the open shell system were studied using the corresponding RASSCF program as an open shell SCF program. For each of the systems studied a full geometry optimization starting from a structure which we believed to be consistent with the experimental observations. The resulting structures are shown in Figs. 6 and 7. At the optimal geometry a vibrational spectrum was calculated using the harmonic approximation. The Hessian was calculated using an analytical procedure. In Table 1 we present the calculated total energies (in a.u.) for the dierent systems studied, together with the formation en-
T. Svensson et al. / Chemical Physics 265 (2001) 323±333 Table 1 Calculated energies (see text) Total energy (H) OH HOO CO2 OCO±HO OCO±HOO
75.421320 150.239574 187.717025 263.140962 337.959965
Energy of formation (kJ/mol)
6.87 8.84
ergy for the complexes in kJ/mol. In Table 6 we show the calculated complex shifts of the fundamental vibrational frequencies for the studied systems. The results will be discussed in connection with the presentation of the experimental results.
4. Assignment 4.1. Nomenclature In complexes of the type studied here, the intramolecular vibrations of the complex components retain their original character in the complex. Therefore, the perturbed ith fundamental of A in a complex with B will be denoted as mi (A±B). When the isotopic composition of B is immaterial, the peroxy radical will be denoted as perox and water as aq.
325
4.1.1. H2 O±CO2 complex The method used to prepare peroxy radicals always produces signi®cant amounts of water [1]. It was therefore necessary to study the water complex of carbon dioxide. The IR spectrum of the matrix isolated water complex of carbon dioxide has as far as we know only been studied in solid nitrogen by Fredin et al. [16] and in solid oxygen by Tso and Lee [17]. There is good qualitative accord between our results and those of Refs. [16,17]. The results are collected in Table 2. When water and carbon dioxide were simultaneously present in the matrix, two bands at 656.0 and 668.0 cm 1 increased signi®cantly relative to the three bands of free CO2 at 662.0, 663.5 and 663.9 cm 1 . These bands were assigned to the complex between H2 O±CO2 . The same band positions were observed with D2 O instead of H2 O. A band near 2340.5 cm 1 was assigned to m3 (O2 C± OH2 ). m1 (H2 O±CO2 ) was observed at 3632.7 cm 1 , m2 (H2 O±CO2 ) at 1593.1 cm 1 and m3 (H2 O±CO2 ) at 3732.9 cm 1 . 4.1.2. HOO±CO2 complexes (1:1) When peroxy radicals and CO2 were codeposited on a matrix we observed a set of new bands close to the peroxy radical and carbon dioxide fundamentals. Based on concentration dependencies we assign the following bands to one or more
Table 2 Observed bands of the complex of water with CO2 in solid argon (cm 1 ) H2 O(D2 O), m1 H2 O(D2 O), m2 H2 O(D2 O), m3 CO2 , m1 CO2 , m2
H2 O
D2 O
CO2
H2 O±CO2
D2 O±CO2
3638.0 1589.1 3734.3
2657.7 1174.6 2771.1
± ± ±
3632.7 1593.1 3732.9
1177.4 2770.9
656.0 668.0
656.0 668.0
662.2a 663.5 663.9
a
CO2 , m3
2345.1a 2339.3 2340.5
2340.5
2340.5
13
2279.5a 2274.0 2275.0
2275.0
2275.0
CO2 , m3
Stable site.
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T. Svensson et al. / Chemical Physics 265 (2001) 323±333
OOH±OCO 1:1 complexes: 3372.1, 3376.4, 1410.1, 1415.6, 1109.5, 1110.9, 656.2, 668.1, 2339.9, 2340.1, 2274.4, 2274.6. In experiments with DOO we assign bands at 2499.5, 2501.7, 1037.9, 1041.8, 655.9, 668.1, 2338.8, 2339.0, 2273.5, 2273.7 to a OOD±OCO complex (1:1). The rate of photolysis varied between the bands. In fact from their photolysis rates they could be assigned to two dierent complexes, I and II. The bands assigned to I had a photolysis rate that was twice as high as bands of
II. We assign the bands at 3376.4, 1415.6, 1110.9, 2339.9 and 2340.1 cm 1 (13 C: 2274.4 and 2274.6 cm 1 ) to I (D: 2501.7, 1041.8, 2338.8, 2339.0) and the bands at 3372.1, 1410.1, 1109.5, 656.2, 668.1 cm 1 to II (D: 2499.5, 1037.9, 655.9, 668.1). The assignments are collected in Table 3. m1 (OOH± OCO) (I, II) at 3376.4 and 3372.1 cm 1 and m1 (OOD±OCO) (I, II) at 2499.5 and 2501.7 cm 1 are shown in Fig. 1. Fig. 2 shows m3 (O 13 CO±HOO) and m3 (O 13 CO±HOO).
Table 3 Observed bands of the complex of the peroxy radical and hydroxyl radical with CO2 (cm 1 ) HOO HOO, m1 HOO, m2 HOO, m3 HO, m CO2 , m2
3413.0 1388.9 1100.9
HO
3548.2a
CO2
662.2b
OH±OCO
3557.0 658.9
HOO±CO2 (I)
HOO±CO2 (II)
3376.4 1415.6 1110.9
3372.1 1410.1 1109.5 656.2 668.1
663.5 663.9 2345.1b
CO2 , m3
2350.1
2339.9 2340.1
2284.5
2274.4 2274.6
OD±OCO
DOO±CO2 (I)
DOO±CO2 (II)
2501.7 1041.8
2499.5 1037.9
2339.3 2340.5 13
2279.5b
CO2 , m3
2274.0 2275.0 DOO DOO, m1 DOO, m2 DOO, m3 DO, m CO2 , m2
2530.2 1020.3 1122.9
DO
2616.1a
CO2
662.2b
2621.4 658.6
655.9 668.1
663.5 663.9 CO2 , m3
2345.1b
2350.7
2338.8 2339.0
2285.0
2273.5 2273.7
2339.3 2340.5 13
CO2 , m3
2279.5b 2274.0 2275.0
a b
Ref. [7]. Stable site.
T. Svensson et al. / Chemical Physics 265 (2001) 323±333
Fig. 1. The HO and DO stretching vibration of the peroxy radical±CO2 complexes; resolution, 0.1 cm 1 ; x-axis, cm 1 ; upper curves, spectra before photolysis, ratioed against background spectra; lower curves, spectra after 60 min of photolysis, ratioed against spectra before photolysis; the curves have been shifted vertically for clarity. (a) m1 (HOO±CO2 ) (II) at 3372.1 cm 1 and m1 (HOO±CO2 ) (I) at 3376.4 cm 1 , (b) m1 (DOO±CO2 ) (II) at 2499.5 cm 1 and m1 (DOO±CO2 ) (I) at 2501.7 cm 1 .
The CO2 bands at 663.5, 663.9, 2339.3, 2340.5, 2274.0 and 2275.0 cm 1 have been assigned to CO2 in a metastable trapping site by Schriver et al. [11]. These bands decrease in intensity during photolysis at the same rate as the bands of complex II. Fig. 2 illustrates this for m3 (O 13 CO), note that the stable site is unaected. When a matrix containing only CO2 was irradiated the bands of the metastable site were unaected. Possibly the photodecomposition of peroxy radicals and peroxy radical complexes deposit enough energy in the matrix to anneal away the metastable site. The
327
Fig. 2. The 13 CO2 , m3 , stretching vibration of the peroxy radical±CO2 complexes; resolution, 0.1 cm 1 ; x-axis, cm 1 ; upper curves, spectra before photolysis, ratioed against background spectra; lower curves, spectra after 60 min of photolysis, ratioed against spectra before photolysis; the curves have been shifted vertically for clarity. The CO2 bands at 2274.0 and 2275.0 cm 1 have been assigned to CO2 in a metastable trapping site by Schriver et al. [11]: (a) m3 (O2 C±OOH) (I) at 2274.4 and 2274.6 cm 1 , (b) m3 (O2 C±OOD) (I) at 2273.5 and 2273.7 cm 1 .
metastable site is also eliminated when the matrix is warmed above 30 K [11]. 4.1.3. The complex between HO and CO2 When matrices containing the OC±HOO complex were irradiated, bands appeared at 658.9, 2350.1, (13 C: 2284.5) and 3557.0 cm 1 . The matrices with matrix isolated peroxy radicals and CO were irradiated in steps, with a spectrum recorded after each step. In order to identify relations
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T. Svensson et al. / Chemical Physics 265 (2001) 323±333
between reactant±product bands in the spectra, correlation coecients were calculated for each pair of bands using standard statistical methods. The M A T L A B â commands corrcoef x; y or corrcoef X were used. If the growth of a product band is due to the decomposition of a precursor, the correlation coecients of their absorption bands are ideally 1. The bands in the OOH±CO complex proved to be anti-correlated with the bands at 658.9, 2350.1 and 3557.0 cm 1 . The correlation coecients were calculated for six matrices with close agreement between the matrices. With H and 0.1 cm 1 resolution the correlation coecients varied between 0.983 and 0.998. In OC±DOO experiments, the corresponding bands appeared at 659.7, 2350.7, (13 C: 2285.0) and 2621.4 cm 1 (Figs. 3±5). The bands at 658.9 and at 2350.1 cm 1 are close to the infrared active fundamentals of carbon dioxide, showing that the photoproduct contains a perturbed CO2 molecule. The 2284.5 cm 1 band is clearly the 13 CO2 band corresponding to 2350.1 cm 1 . Note the small but signi®cant H- to D-shifts of the carbon dioxide fundamentals which show that the carbon dioxide molecule is perturbed by a species which contains hydrogen. Since the growth rate suggests that the photoproduct forms from OC±HOO it seems clear that the perturbing species is a hydroxyl radical. The position of the third band, 3557.0 cm 1 (D: 2621.4 cm 1 ) is compatible with this assignment. 4.1.4. Unassigned bands When matrices with matrix isolated peroxy radicals and CO were irradiated we observed weak bands at 1862.5 and 1863.8 cm 1 (D: 1925.0, 849.4 cm 1 ). These bands are probably due to HCO [14]. A band near 1903.7 cm 1 (1905.2 cm 1 in Dexperiments) was formed with a high rate in CO2 HOO experiments. It is possibly due to the complex HO±CO3 . Weissberger et al. [15] observed a set of new bands when solid CO2 matrix with dispersed O3 was irradiated with UV light. The new bands were assigned to CO3 . Traces of O3 are always present in our matrices, possibly since O atoms form from H2 O present in the microwave discharge. Bands which we assign to CO3 were observed at 2043.4 (strongest) and 2037.3 cm 1 .
Fig. 3. The HO and DO stretching vibration of the hydroxy radical±CO2 complex; resolution, 0.1 cm 1 ; x-axis, cm 1 ; upper curves, spectra before photolysis, ratioed against background spectra; lower curves, spectra after 60 min of photolysis, ratioed against spectra before photolysis; the curves have been shifted vertically for clarity. (a) m(OH±OCO) at 3557.0 cm 1 , (b) m(OD± OCO) at 2621.4 cm 1 .
During the ®rst minutes of photolysis, the bands grow at a high rate. It is followed by a growth at slower rate. Small amounts of CO3 were also observed by irradiating a matrix with codeposited peroxy radicals and CO. In addition to the bands assigned to the peroxy radical and the hydroxy radical complexes, we observed a number of relatively weak bands with H- to D-shifts near the fundamentals of HOO, HO and CO2 which are not assigned. Most of them were also observed in experiments with codeposition of peroxy radicals and CO2 . They are collected in Table 4.
T. Svensson et al. / Chemical Physics 265 (2001) 323±333
Fig. 4. The CO2 , m2 , stretching vibration of the hydroxy radical±CO complex; resolution, 0.1 cm 1 ; x-axis, cm 1 ; upper curves, spectra before photolysis, ratioed against background spectra; lower curves, spectra after 60 min of photolysis, ratioed against spectra before photolysis; the curves have been shifted vertically for clarity. (a) m2 (OCO±HO) at 658.9 cm 1 , (b) m2 (OCO±DO) at 658.6 cm 1 .
By irradiating (at 266 nm) the matrix with codeposited HOO and CO2 , we observed a set of bands that grew near the HOO vibrational fundamentals of the HOO±CO2 complexes. After approximately 30 min of irradiation they started to photodecompose. The bands were observed at 3370.9 cm 1 (2498.5 cm 1 with DOO), at 1411.1 cm 1 (1039.1 cm 1 with DOO) and at 1108.4 cm 1 (no corresponding band was observed with DOO).
329
Fig. 5. The 13 CO2 , m3 , stretching vibration of the hydroxy radical±CO2 complexes; resolution, 0.1 cm 1 ; x-axis, cm 1 ; upper curves, spectra before photolysis, ratioed against background spectra; lower curves, spectra after 60 min of photolysis, ratioed against spectra before photolysis; the curves have been shifted vertically for clarity. The band at 2279.5 cm 1 is due to 13 CO2 (stable site); the band at approximately 2281.5 cm 1 , see Table 4: (a) m3 (O 13 CO±HO) at 2284.5 cm 1 , (b) m3 (O 13 CO±DO) at 2285.0 cm 1 .
We observed a set of bands that grew in the HO and CO2 (m2 and m3 ) regions in experiments with matrix isolated peroxy radicals and CO or CO2 . They were observed at 3428.9, 655.1, 666.1, 2346.6 (13 C: 2281.3) cm 1 (DOO: 2543.3, 654.8, 666.2, 2346.8 (13 C: 2281.4 cm 1 )). The growth rate was approximately the same for the bands and anticorrelated with the decrease of the peroxy radical bands. It is clear that the bands at 655.1 and 666.1 belong to the same specie but we could not conclude de®nitively that the bands at 3428.9, 2346.6
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T. Svensson et al. / Chemical Physics 265 (2001) 323±333
Table 4 Observed bands with H- to D-shifts H (cm 1 )
D (cm 1 )
Experiment
Notes
2336.5 2335.4a
2335.9 2335.0a
Peroxy radicals CO2 ''
Photolysis rate: >photolysis rate (HOO±CO2 ) ''
2334.4 2337.1 3370.9
2334.5 2336.4 2498.5
Peroxy radicals CO2 Peroxy radicals CO Peroxy radicals CO2
1411.1 1108.4
1039.1
'' ''
Photolysis rate:
3428.9
2543.3
655.1 666.1 2346.7 2281.3
654.8 666.2 2346.8 2281.4
Peroxy radicals CO2 and hydroxyl radicals CO2 '' '' '' ''
2342.1
2342.0
2276.7
2276.6
b
3382
b
2507
Grow when the matrix is irradiated '' '' '' ''
Peroxy radicals CO2 and hydroxyl radicals CO2 ''
Grow when the matrix is irradiated ''
Peroxy radicals CO2
Photolysis rate: photolysis rate (HOO±CO2 )
a
Satellite. b Broad band.
and 2281.3 cm 1 also belong to this specie. One band at 2342.1 (13 C: 2276.7) cm 1 (DOO: 2342.0 (13 C: 2276.6) cm 1 ) was formed in the same experiments as the set of bands above, but at a somewhat higher rate. A band at 3382 cm 1 (2507.1 cm 1 in Dexperiments) was only observed when peroxy radicals and CO2 were present at the same time in the matrix and obviously it is a product formed by codepositing peroxy radicals and carbon dioxide (Fig. 2). Its concentration dependence diered from the 1:1 complex bands. The photolysis rate was approximately half of the rate of HOO±CO2 (II). A band at 2336.5 cm 1 was observed, with a small satellite at 2335.4 cm 1 . This band was almost eliminated from the matrix by 5 min of irradiation (266 nm). In D-experiments the band was observed at 2335.9, with a small satellite at 2335.0 cm 1 . The position of the band and the sensitivity to photolysis indicate the presence of CO2 and O3 . The band could be due to a three component species like water±CO2 ±O3 .
5. Discussion 5.1. H2 O±CO2 complex The results from the study of the water±CO2 complex is in agreement with an earlier studies [16,17]. We observe the splitting of the bending mode in CO2 (12.0 cm 1 ). This is consistent with a structure where the water oxygen binds to the carbon dioxide carbon, as found by Peterson and Klemperer [18] and earlier predicted by J onsson et al. [19]. Note also that the antisymmetric stretch of carbon dioxide is shifted towards lower frequencies. In the OCO±HCl complex, where HCl forms a hydrogen bond to a carbon dioxide oxygen, the antisymmetric stretch is blue shifted. The shifts of the water fundamentals are typical for a complex with water as a lone pair donor. 5.2. Peroxy radical±CO2 complexes HOO±CO2 (I, II): The complex shifts of HOO in the two OCO±HOO complexes dier by rather
T. Svensson et al. / Chemical Physics 265 (2001) 323±333
331
Table 5 Bands of some selected CO2 complexes (cm 1 ) Free
ClH±OCOa
HCl, m HO, me HOO, m1 HOO, m2 HOO, m3 H2 O, m1 H2 O, m2 H2 O, m3 CO2 , m1 CO2 , m2
2870.8 3548.2 3413.0 1388.9 1100.9 3638.0 1589.1 3734.3
2854.1
662.2
658
656.0 668.0
658.9
CO2 , m3 13 CO2 , m3
2345.1 2279.5
2346.1 2280.9
2340.5 2275.0
2350.1 2284.5
d
H2 O±CO2 b
OH±OCOc 3557.0
3632.7 1593.1 3732.9
HOO±CO2 (I)c
HOO±CO2 (II)c
3376.4 1415.6 1110.9
3372.1 1410.1 1109.5
656.2 668.1 2274.4 2274.6
a
Ref. [18]. Taken from Table 2. c Taken from Table 3. d Ref. [18]. e Ref. [7]. b
insigni®cant amounts (Table 3). The only dierence between the two complexes seems to be their photodecomposition rates. We therefore believe that they are identical complexes, trapped in different sites. The bending vibration of CO2 bound to a peroxy radical splits into two components in the same way as in the carbon dioxide water complex (Table 5). The water carbon dioxide complex is a planar C2v complex, with the water oxygen close to the carbon dioxide carbon [18]. We therefore expect that the terminal oxygen of HOO is close to the carbon dioxide carbon. We note that for the linear OCO±HCl complex, the carbon dioxide bend is a single band at 658 cm 1 [20]. The OH shifts of the two complexes, 36.6 and 40.9 cm 1 , are approximately twice the HCl shift of OCO±HCl. The observation that the OH shift is larger than the HCl shift of the CO2 ±HCl complex supports the assumption of an additional interaction in the HOO complex [3]. The complexes are thus expected to have a cyclic structure, probably planar, with the peroxy radical hydrogen bonded to a lone pair of an oxygen atom and turned around its HO bond so that its terminal oxygen is as close as possible to the carbon dioxide carbon.
As can be seen from Fig. 6, the calculated structure of the peroxy radical carbon dioxide complex has a hydrogen bond from the peroxy radical to a carbon dioxide oxygen. The terminal
Fig. 6. The calculated structure of the peroxy radical carbon dioxide complex (see text).
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T. Svensson et al. / Chemical Physics 265 (2001) 323±333
Table 6 Calculated and experimental complex shifts (cm 1 ) (OCO± HOO type II was used with one exception) Calculated m2 (OCO±HOO) m3 (OCO±HOO) m1 (HOO±OCO) m2 (HOO±OCO) m3 (OOH±OCO) m(OH±OCO) m2 (OCO±HO) m3 (OCO±HO) a
2.7 2.9
Experimental 5.9 6.0
2.0 10.7 25.9 3.9
5.0 40.9 21.2 8.6
0.0 3.5 4.3
8.8 3.3 5.0
a
From type OCO±HOO type I.
oxygen of the peroxy radical is turned in the direction of the carbon dioxide. The calculated structure is thus in general agreement with the structure suggested by the observed complex shifts. The calculated complex shifts (Table 6) are in approximate agreement with the observed shifts but the calculated split of the carbon dioxide is almost zero, suggesting a too weak interaction between the terminal oxygen of the peroxy radical and the carbon dioxide carbon. A similar problem was encountered in a calculation on the interaction between a peroxy radical and chlorine dioxide [21]. Here the calculation produced a too strong hydrogen bond from the peroxy radical to an oxygen atom but a too weak interaction between the terminal oxygen of the peroxy radical and the chlorine atom. From theoretical considerations one would in general expect a too weak carbon±oxygen interaction in the calculations, since the attractive dispersion interaction is lacking in the SCF approximation. 5.3. The hydroxyl radical±CO2 complex OH±OCO: The hydroxyl radical carbon dioxide complex has a single strong OCO bend at 658.9 cm 1 , which shifts to 658.6 cm 1 in D-experiments. The antisymmetric stretch of OCO is blue shifted 5.0 cm 1 with respect to the major site fundamental of carbon dioxide. The spectral changes of complexed carbon dioxide are thus very similar to the shifts of carbon dioxide complexed with HCl
Fig. 7. The calculated structure of the hydroxyl radical carbon dioxide complex (see text).
(Dm2 4 cm 1 and Dm3 1:0 cm 1 ). We therefore believe that the complex is close to linear with the hydroxyl radical forming a weak hydrogen bond to a carbon dioxide oxygen. For this complex structure one expects a signi®cant red shift of the hydroxyl radical fundamental. Instead we observe a small blue shift. The exact position of the hydroxyl radical fundamental in solid argon is uncertain, since there is a possibility that the observed band is due to hydroxyl radicals interacting weakly with some impurity. The size of the shift is therefore uncertain but it is at most a few wavenumbers. As can be seen from Fig. 7, the ab initio calculation predicts an almost linear complex, in agreement with the discussion above. The calculated complex shifts are quite close to the observed. Even the most surprising experimental result, the practically nonexistent shift of the hydroxyl radical, is reproduced. We ®nally note that in the HO±HF complex [10], where OH accepts a hydrogen bond from HF, the OH stretching frequency is red shifted by 10.3 cm 1 . Acknowledgements This work was supported by the Swedish Natural Science Research Council. The Carl Trygger Foundation ®nanced an upgrading of the FTIR Spectrometer. The YAG Laser was ®nanced by a grant from FRN. References [1] [2] [3] [4]
B. Nelander, J. Phys. Chem. A 101 (1997) 9092. A. Engdahl, B. Nelander, Chem. Phys. 249 (1999) 215. T. Svensson, B. Nelander, Chem. Phys. 262 (2000) 445. M.W. Todd, D.T. Todd, M.I. Lester, J. Phys. Chem. A 104 (2000) 6532. [5] J. Goodman, L.E. Brus, J. Chem. Phys. 67 (1977) 4858.
T. Svensson et al. / Chemical Physics 265 (2001) 323±333 [6] N. Acquista, L.J. Schoen, D.R. Lide Jr., J. Chem. Phys. 48 (1968) 1534. [7] B.M. Cheng, Y.P. Lee, J.F. Ogilvie, Chem. Phys. Lett. 151 (1988) 109. [8] L. Khriachtchev, M. Pettersson, S. Touminen, M. R as anen, J. Chem. Phys. 107 (1997) 7252. [9] M. Fushitani, T. Shida, T. Momose, M. Rasanen, J. Phys. Chem. 104 (2000) 3635. [10] T.C. McInnis, L. Andrews, J. Phys. Chem. 96 (1992) 2051. [11] A. Schriver, L. Schriver-Mazzuoli, A.A. Vigasin, Vibr. Spect. 23 (2000) 83. [12] K. Andersson, M.R.A. Blomberg, M.P. Fulscher, G. Malmqvist, P. Neogrady, Karlstr om, R. Lindh, P.-A. B.O. Roos, A.J. Sadlej, L. Seijo, L. Serrano-Andres, P.E.M. Siegbahn, P.O. Widmark, M O L C A S , version 4.0., Department of Theoretical Chemistry, Lund University, Sweden, 1997.
333
[13] K. Pierloot, B. Dumez, P.-O. Widmark, B.O. Roos, Theor. Chim. Acta 77 (1990) 291. [14] M.E. Jacox, Vibrational and electronic energy levels of polyatomic transient molecules, J. Phys. Chem. Ref. Dat Monogr., New York 3 (1994). [15] E. Weissberger, W.H. Breckenbridge, H. Taube, J. Chem. Phys. 47 (1967) 1764. [16] F. Fredin, B. Nelander, G. Ribbeg ard, Chemica Scripta 7 (1974) 11. [17] T.-L. Tso, E.K.C. Lee, J. Phys. Chem. 89 (1985) 1612. [18] K.I. Peterson, W. Klemperer, J. Chem. Phys. 80 (1984) 2439. [19] B. J onsson, G. Karlstr om, H. Wennerstr om, Chem. Phys. Lett. 30 (1975) 58. [20] N. Fourati, B. Silvi, J.P. Perchard, J. Chem. Phys. 81 (1984) 4737. [21] T. Svensson, B. Nelander, A. Bernhardsson, G. Karlstr om, J. Phys. Chem. A 103 (1999) 4432.